Gradient amplifiers generate magnetic coil currents to provide spatial information in MRI systems. State of the art gradient amplifiers use fast switching power devices with analogue control (e.g. PID controller) of the half bridges to provide the desired duty cycle with high accuracy. Such a gradient amplifier is known from U.S. Pat. No. 7,253,625. This solution is very cost-intensive.
There are mainly two effects making the use of cheaper components difficult, since the accuracy of the duty cycle is reduced:
First, the converter for the gradient amplifier consists typically of one or more full bridges, each full bridge being composed of two half bridges. A half bridge is a series connection of two power switches comprising e.g. MOSFETs or IGBTs, and is connected to the mains supply. The output voltage of the half bridge is tapped at the centre point between the power switches. Assuming that the negative rail of the supply is at zero voltage the half bridge can supply a positive voltage, when the upper switch is switched on while the lower switch is switched off, and a zero voltage, when the lower switch is switched on while the upper switch is switched off. In order to avoid a short circuit when a transition from positive to zero voltage or vice versa takes place, a certain waiting time is required after the first switch is switched off before the second switch is switched on. This time is the so-called dead-time. During this dead-time the output voltage is not determined by the switch states but depends on the output current and internal states of the system and parasitics. Thus, there is a certain effective voltage error induced by the dead-time, that can hamper the control performance or at the worst lead to controller instabilities. The required dead time increases with an increasing switching time of the power switches, resulting in an increased effective voltage error.
Second, even when a power switch is switched on, a voltage drop over the terminals of the switch occurs. This voltage drop depends nonlinearly on the switch current and causes also a voltage error that can lead to similar problems as the dead-time. This error is referred to as forward voltage and increases with the switch current.
State of the art methods to compensate for these voltage errors assume a highly inductive load, so that the current changes only very slowly over one PWM period. This provides a simple compensation, as the voltage error is strongly influenced by the current which can be assumed to be nearly constant over one PWM period. However, the load of the gradient amplifier consists typically of an output filter and a gradient coil. The output filter typically is a LC-filter, which reduces interferences and ripple-currents. While the inductance of the gradient coil is very high, the filter inductance is typically low to minimize voltage loss over the filter. This causes highly dynamic load currents for the converter so that even during one PWM period it can have several zero crossings.